A comparison using the caesium-137 technique of the relative importance ofcultivation and overland flow on soil erosion in a steep semi-tropical sub-catchment (vol 39, pg 219, 2001)

The spatial pattern of net soil loss on 6 downslope transects in a small se mi-tropical sub-catchment was measured in 1990-91 using the resident caesiu m-137 deficit technique. The sub-catchment consisted of 2 opposing hillslop es which shed water to an intermittent stream in the valley bottom of the s ub-catchment. There were 3 transects on each of the opposing hillslopes, an d measurement indicated net soil loss from all 6 transects. Furthermore, th e spatial pattern of caesium-137 deficit did not indicate the accumulation of soil expected due to the slope decrease toward the bottom of the valley. Possible explanations of this finding could be the effect of periodic floo ding of the intermittent valley stream, or seepage-accelerated erosion. Pineapple cultivation in the sub-catchment since 1950 included intensive cu ltivation at 4-year intervals by downslope-moving rotary hoe. The paper dev elops a theoretical prediction of the spatial pattern of net soil loss expe cted due to such cultivation, as well as the expected pattern of soil loss due to overland flow on the hillslopes. The spatial patterns of soil loss d ue to these 2 different soil erosion mechanisms were then compared with the pattern of net soil loss indicated by caesium-137 depletion to provide an assessment of their likely relative importance in contributing to soil loss . In the upper part of each hillslope, this comparison of spatial trends di d not allow the dominant cause of soil erosion to be distinguished. Both th e model of erosion due to cultivation and that due to hillside overland flo w predicted soil accumulation in the lower valley sides where slope decreas ed. Neither model represented the net loss of such accumulated soil indicat ed by caesium-137 deficit, and this loss possibly occurred during periodica lly observed flooding of the valley floor, or due to surface burial with ca esium-137 depleted subsoil.